1. (especially of medicine) of disease or anything resembling a disease; attacking or affecting many individuals in a community or a population simultaneously
an epidemic outbreak of influenza
2. Common to, or affecting at the same time, a large number in a community; -- applied to a disease which, spreading widely, attacks many persons at the same time; as, an epidemic disease; an epidemic catarrh, fever, etc. See Endemic.
3. a widespread outbreak of an infectious disease; many people are infected at the same time